1. Field of the Invention
The present invention relates to an optical transmission system for frequency modulating frequency-division-multiplexed multiplexed-channel signals as a single unit for optical transmission and optical transmission apparatus used for the system.
2. Description of the Related Art
Conventionally, there is known a system for frequency modulating frequency-division-multiplexed multiplexed-channel signals as a single unit for transmission. FIG. 10 is a block diagram showing the configuration of such a conventional optical transmission system. In FIG. 10, an optical transmission system 50 has a configuration in which a transmitter 51 connects to a receiver 53 through an optical transmission link, such as an optical fiber 52.
The transmitter 51 is composed of a general FM modulator 54 for frequency modulating a carrier signal as a single unit by frequency-division-multiplexed multiplexed-channel signals and an optical modulator 55 for modulating the intensity state of a signal light by the resulting frequency-modulated signals. The receiver 53 is composed of an optical receiver for performing photoelectric conversion of transmitted optical signals and an FM demodulator 57 for frequency demodulating the frequency-modulated signal to output frequency-division-multiplexed multiplexed-channel signals.
Next, the operation of a conventional optical transmission system is described below. Frequency-division-multiplexed multiplexed-channel signals input to the FM modulator 54 are frequency modulated as a single unit. The resulting frequency-modulated signal is applied to the optical modulator, which changes the intensity state of the signal light, and is transmitted through the optical fiber line 52. Optical signal transmitted to the receiver 53 undergo photoelectric conversion, and the resulting frequency-modulated signals is frequency demodulated by the FM demodulator 57 to restore frequency-division-multiplexed multiplexed-channel signals.
An optical transmitter used for this type of optical transmission systems is disclosed, for example, in JP-A-8-274714 (optical signal transmitter). This optical signal transmitter can transmit multiplexed-channel signals (video signals) by converting amplitude modulated broadband electrical signals to frequency-modulated electrical signals, and thus can provide an optical transmitter having good signal quality.
However, in this type of conventional optical transmission system and an optical transmitter used for the optical transmission system, there occurs a ripple-shaped group delay deviation which periodically varies from the specific frequency shown in FIG. 3. This group delay deviation raises the problems such that the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter becomes small, that is to say, bandwidth of FM signal frequency modulated as a single unit is reduced so that distortion caused by the ripple-shaped group delay deviation comes to be larger distortion in case of the signals being transmitted from the transmitter to the receiver.
The object of the invention is to provide an optical transmission system for increasing the level per waveform of multiplexed-channel signals and increasing the bandwidth of FM signal to reduce the distortion caused by a ripple-shaped group delay deviation in a transmitter, a receiver and optical transmission link, even in the case where the number of channels for frequency-division-multiplexed (FDM) multiplexed-channel signal input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
An optical transmission system according to the invention comprises a transmitter for transmitting optical signals based on the input frequency-division-multiplexed multiplexed-channel signals and a receiver for receiving optical signals transmitted from the transmitter through optical transmission link, the transmitter having signal level adjusting means for reducing the distortion caused by a ripple-shaped group delay deviation in a transmitter a receiver and the optical transmission link in transmitting optical signals from a transmitter or a receiver. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation in a transmitter, a receiver and optical transmission link can be reduced by adjusting to increase the level per waveform of multiplexed-channel signals through signal level adjusting means, even in the case where the number of channels for the frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmission system according to the invention, the transmitter is equipped with a general FM modulator for frequency modulating multiplexed-channel signals as a single unit and the receiver is equipped with optical modulation means for adjusting the intensity state of the signal light through frequency modulated signals obtained by frequency modulating multiplexed-channel signals as a single unit. Further equipped is an optical receiver for performing photoelectric conversion of the optical signal transmitted to the receiver through the optical transmission link on a general FM demodulation for frequency demodulating FM signal obtained thereby. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation in a transmitter, a receiver and optical transmission link can be reduced by increasing the level per waveform of multiplexed-channel signals through signal level adjusting means when signals are transmitted from the transmitter to the receiver through an optical channel, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmission system according to the invention, the signal level adjusting means is equipped with an amplifier for amplifying multiplexed-channel signals and an attenuator for attenuating while adjusting the output of the amplifier. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation in a transmitter, a receiver and optical transmission link can be reduced by increasing the level per waveform of multiplexed-channel signals based on the output of the amplifier when FDM multiplexed-channel signals are transmitted from the transmitter to the receiver through an optical link, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmission system according to the invention, the signal level adjusting means is used to manually adjust the input level of the FDM multiplexed-channel signals based on the number of channels for the FDM multiplexed-channel signals. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation of a transmitter, a receiver and optical transmission link can be reduced by manually adjusting the level in waveform of FDM multiplexed-channel signals when they are transmitted from the transmitter to the receiver through the optical link, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmission system according to the invention, the transmitter comprises a bandwidth detector for detecting the bandwidth of the FM signal. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation of signals transmitted from a transmitter, a receiver and optical transmission link can be reduced by detecting the bandwidth of FM signal whose input level is adjusted to enlarge the bandwidth of the FM signal when signals are transmitted from the transmitter to the receiver through an optical channel, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
An optical transmitter according to the invention comprises a transmitter for transmitting optical signals based on the input frequency-division-multiplexed multiplexed-channel signals, the transmitter comprising signal level adjusting means for reducing the distortion caused by a ripple-shaped group delay deviation in the transmitter, the receiver and the optical transmission link in transmitting optical signals from the transmitter to the receiver, and for adjusting the level of transmit signals depending on the increase/decrease in the number of channels for FDM multiplexed-channel signals to keep constant of the bandwidth of the FM signal. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation of signals transmitted from a transmitter to a receiver can be reduced to transmit the multiplexed-channel signals with good signal quality, by adjusting to increase the level per waveform of multiplexed-channel signals through signal level adjusting means, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmitter according to the invention, the transmitter is equipped with a general FM modulator for frequency modulating the multiplexed-channel signals as a single unit. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation of signals transmitted from a transmitter to a receiver can be reduced to transmit multiplexed-channel signals with good signal quality, by increasing the level per waveform of multiplexed-channel signals when signals are transmitted from the transmitter to the receiver through an optical channel, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmitter according to the invention, the signal level adjusting means is equipped with an amplifier for amplifying the multiplexed-channel signals and an attenuator for attenuating while adjusting the output of the amplifier. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation of signals transmitted from a transmitter to a receiver can be reduced by increasing the level per waveform of multiplexed-channel signals based on the output of the amplifier when signals are transmitted from the transmitter to the receiver through an optical channel, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmitter according to the invention, the signal level adjusting means is used to manually adjust the input level of multiplexed-channel signals based on the number of channels for the multiplexed-channel signals. Through such a system configuration, the bandwidth of the FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation of signals transmitted from a transmitter to a receiver can be reduced by manually adjusting the level per waveform of FDM multiplexed-channel signals when they are transmitted from the transmitter to the receiver through an optical transmission link, even in the case where the number of channels for frequency-division-multiplexed FDM multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.
In an optical transmitter according to the invention, the transmitter comprises a bandwidth detector for detecting the bandwidth of FM signal. Through such a system configuration, the bandwidth of FM signal can be enlarged and the distortion caused by a ripple-shaped group delay deviation in a transmitter, a receiver and the optical transmission link can be reduced by detecting the bandwidth of FM signal whose input level is adjusted to enlarge the bandwidth of the FM signal when FDM signals are transmitted from the transmitter to the receiver through an optical transmission link, even in the case where the number of channels for frequency-division-multiplexed multiplexed-channel signals input to the transmitter is small, and the bandwidth of FM signal frequency modulated as a single unit is reduced.